The present invention relates to a lens body tube having a zooming and focusing functions, and more particularly to a lens body tube appropriately applicable to a for-element focusing mechanism.
A for-element focusing type lens body tube, in which a front lens group moves for a zooming or focusing operation, is designed such that the front lens group is not caused to rotate around the optical axis, to keep accessories attached to a lens frame such as a filter and a hood from rotating around the optical axis. For example, Japanese Unexamined Patent Publication No. HEI11-84203 proposes a structure that inhibits the lenses inside the lens body tube including the front lens group from rotating around the optical axis when performing the zooming, but that allows the lenses to move along the optical axis maintaining the angular position around the optical axis, when performing the focusing. However, the technique disclosed in Japanese Unexamined Patent Publication No. HEI11-84203 does not allow the front lens group alone to move along the optical axis when performing the focusing, and besides, fails to specifically describe a structure of a plurality of rings included in the lens body tube.
FIG. 6 shows an example of a conventional for-element focusing type lens body tube. This lens body tube includes a main tube 1 composed of an inner tube 11 and an outer tube 12 combined with screws 13, and the inner tube 11 accommodates a plurality of lenses, in this case, two lens groups of a front lens group 2 and a rear lens group 3, installed movably along the optical axis. On the outer circumference of the outer tube 12, a zoom ring 4 and a focus ring 5 are rotatably mounted around the optical axis, and also a cam ring 6 is mounted on the outer circumference of the inner tube so as to rotate around the optical axis, interlocked with the zoom ring 4. Hereinafter, the term related to “rotation” is to be construed as describing the rotation around the optical axis. A lens frame 31 of the rear lens group 3 is engaged with an inner cam 61 of the cam ring 6, so that rotation of the cam ring 6 causes the rear lens group 3 to move along the optical axis. To the outer circumference of the cam ring 6, a moving ring 7A which is engaged with an outer cam projection 62 of the cam ring 6 is fitted, so as to move in the optical axis direction with the rotation of the cam ring 6, and also a helicoid ring 8A is fitted to the outer circumference of the moving ring 7A, so as to move together with the moving ring 7A in the optical axis direction when the focus ring 5 is rotated. The helicoid ring 8A is provided with a male helicoid (spiral groove) 84 on an outer circumferential surface thereof. Further, a sleeve portion 24, provided in the optical axis direction around the outer circumference of a lens frame 21 of the front lens group 2, overlaps the outer circumference of the helicoid ring 8A, such that a female helicoid 25 formed on an inner surface of the sleeve portion 24 is screwed together with the male helicoid 84 of the helicoid ring 8A. The lens frame 21 of the front lens group 2 is fitted to a key groove (not shown) provided in the optical axis direction on the inner surface of the inner tube, and is hence kept from rotating inside the inner tube.
In the conventional lens body tube thus configured, rotating the zoom ring 4 causes the cam ring 6 to rotate, so that the inner cam 61 of the cam ring 6 causes the rear lens group 3 to move along the optical axis, in a zooming operation. Here, the outer cam 62 also causes the moving ring 7A, and hence the helicoid ring 8A to move in the optical axis direction, which in turn causes the front lens group 2, which is screwed together with the helicoid ring 8A via the sleeve portion 24, to move along the optical axis. Such operation allows the front lens group 2 and the rear lens group 3 to simultaneously move over a predetermined distance along the optical axis, thus completing the zooming action. Likewise, in a focusing operation, rotating the focus ring 5 causes the helicoid ring 8A to interlockedly rotate. At this moment, the moving ring 7A is kept from being rotated because of the key structure (not shown) provided in the optical axis direction, and from moving in the optical axis direction because of being engaged with the outer cam projection 62 of the cam ring 6. Thus the rotation of the helicoid ring 8A causes the sleeve portion 24 of the lens frame 21 screwed together with the helicoid ring 8A, and hence the front lens group 2 to move along the optical axis, thereby achieving the desired focusing effect. In either of the zooming and focusing operation, the front lens group 2 is kept from being rotated because of the key structure via which the front lens group 2 is engaged with the inner tube 11. Consequently, when accessories such as a filter and a hood (not shown) are attached to the lens frame 2 of the front lens group 2, those accessories are kept from rotating during the zooming or focusing operation.
In the lens body tube shown in FIG. 6, the front lens group 2 has to be moved along the optical axis independently in the zooming and focusing operations. To achieve this, the moving ring 7A is interposed between the helicoid ring 8A interlocked with the focus ring 5 and the cam ring 6 interlocked with the zoom ring 4, to thereby eliminate interference between the helicoid ring 8A and the cam ring 6 when rotating. In other words, the moving ring 7A does not have to be rotated by the rotational movement of the helicoid ring 8A in the focusing operation, and yet has to transmit the movement in the optical axis direction caused by the cam ring 6 to the helicoid ring 8A, in the zooming operation. Besides, for achieving the helicoidal engagement between the helicoid ring 8A and the lens frame 21 of the front lens group 2 via the sleeve portion 24, the associated components have to be radially stacked. More specifically, the three ring-shaped components, namely the moving ring 7A, the helicoid ring 8A and the sleeve portion 24 of the front lens group 2, have to be radially stacked on the outer circumference of the cam ring 6, which naturally leads to an increase in diameter in this portion and thus constituting a bottleneck in making the lens body tube smaller in diameter.